The invention relates to gas separation by means of membranes, and in particular to membrane processes used to separate organic compounds from gas mixtures.
Gas streams containing organic compounds, such as light saturated and unsaturated hydrocarbons, are commonplace in the petrochemical industry, in gas and oil fields, and in refineries.
It has been known for many years that membranes can be used to treat such streams, to separate one or more of the organic components from one or more of the other gases, such as air, nitrogen, methane or hydrogen. A separation of this type requires a membrane to be selective either in favor of the organic component or in favor of the other gas. Membranes that use rigid glassy polymers as the selective material are generally selective in favor of smaller molecules over larger molecules, based on the faster diffusion of the small molecules, and thus tend to be selective for permanent gases and methane over organic compounds. Membranes that use elastomeric polymers as the selective material are generally selective in favor of larger, more condensable molecules over smaller molecules, based on the greater solubility of the large molecules in the polymer, and thus tend to be selective for organic compounds over permanent gases and methane.
U.S. Pat. Nos. 4,553,983; 4,857,078; 4,963,165; 4,906,256; 4,994,094; 5,032,148; 5,069,686; 5,127,926; 5,281,255 and 5,501,722 all describe such organic-selective membranes, and systems or processes using them.
It has also been known for many years that condensation and membrane separation may be combined into an integrated process for separating organic components from gas streams. This may be done, for example, by passing the uncondensed gas from the condensation step to a membrane separation step. If the membrane separation step produces an organic-enriched permeate, this permeate may be recirculated to the condensation step to increase the recovery of the organic component. U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843; 5,374,300; 5,755,855; 5,785,739 and 5,769,927 all describe such processes.
It is further known that membrane separation processes can be carried out as single-stage or as multistage operations, with either the residue or the permeate streams, or both, from the first stage being sent to one or more additional membrane separation steps to increase the purity of those respective streams. For example, amongst the patents cited above, U.S. Pat. Nos. 5,089,033 and 5,199,962 show two-step configurations in which the residue from the first membrane separation step is passed for further treatment to a second membrane separation step.
Membrane processes are also used commercially for other gas separations, including separation of nitrogen from air, and of hydrogen from nitrogen. In these separations, both gases of the pair to be separated are permanent gases, that is, neither component is condensable except under cryogenic conditions of very high pressure and very low temperature. In this case, glassy polymer membranes, which preferentially permeate the smaller gas molecules on the basis of their faster diffusion through the polymer, are used. Separations using glassy membranes may also be performed as multistage operations, and may be used in conjunction with non-membrane separation operations. U.S. Pat. No. 4,717,407 discloses a two-stage membrane process combined with cryogenic separation to recover helium from a gas stream. U.S. Pat. No. 4,180,388 discloses a two-step membrane separation process in which the pressure ratio across the first step is lower than the pressure ratio across the second step. U.S. Pat. No. 4,180,552 discloses a process for hydrogen recovery from ammonia synthesis purge gas. The process involves separating hydrogen from nitrogen by means of a two-step membrane separation process. The two membrane separation steps are operated at different pressure ratios, as in the ""388 patent, and the two permeates are recirculated to compressors in series used to compress the feed gases to the reactor.
Despite the extensive literature, designing an energy and cost-efficient process for any particular separation remains difficult, because performance can be influenced by many factors, including intrinsic selectivity of the membrane, flux of the membrane, membrane area used, pressure difference between feed and permeate sides, pressure ratio between feed and permeate sides, pressure drop along the membrane modules, concentration of feed and recycle streams, concentration polarization effects, availability and cost of power, presence of other components in the feed, temperature, potential for fluctuations in feed flow and composition, and availability of suitable destinations for waste streams. Depending on the type of separation that is to be carried out, the interactions of at least some of these factors must be considered during process design, and an acceptable balance between them, consistent with other separation-specific considerations, must be obtained.
The invention is an improved process for treating gas mixtures containing at least an organic component and a second gas. The process separates the organic component from the second gas and provides discrete product streams enriched in each. The process may be configured so that either or both of these streams are produced in a purity sufficient for direct reuse in the upstream operation that produces the feed gas mixture.
The process includes a compression step combined with a membrane separation step.
Both the compression and the membrane separation are themselves performed in at least two sub-steps. The membrane separation steps are carried out using membranes that are selective in favor of the organic component over the second gas. The steps are integrated so that the organic-enriched permeates from the two membrane separation steps are recirculated separately to the two compressor stages.
In a basic embodiment, the process of the invention includes the following steps:
(a) passing a gas stream, comprising an organic component and a second gas, through a first compression stage and a second compression stage in series, thereby forming a compressed stream;
(b) withdrawing at least a portion of the organic component from the compressed stream as a withdrawn stream, thereby leaving a remaining stream;
(c) performing a first membrane separation step, comprising:
(i) providing a first membrane having a first feed side and a first permeate side;
(ii) passing the remaining stream over the first feed side under conditions in which there is a pressure drop between the first feed side and the first permeate side;
(iii) withdrawing from the first feed side a first residue stream enriched in the second gas compared with the remaining stream;
(iv) withdrawing from the first permeate side a first permeate stream enriched in the organic component compared with the remaining stream;
(d) performing a second membrane separation step, comprising:
(i) providing a second membrane having a second feed side and a second permeate side;
(ii) passing the first residue stream over the second feed side under conditions in which there is a pressure drop between the second feed side and the second permeate side;
(iii) withdrawing from the second feed side a second residue stream enriched in the second gas compared with the first residue stream;
(iv) withdrawing from the second permeate side a second permeate stream enriched in the organic component compared with the first residue stream;
(e) recirculating at least a portion of the first permeate stream to the second compression stage;
(f) recirculating at least a portion of the second permeate stream to the first compression stage.
The invention differs from prior art compression/membrane hybrid processes for separating organic components from other gases in that the permeate from the first membrane step is recirculated to the second compression stage and the permeate from the second membrane step is recirculated to the first compression stage. This method reduces the load on the first compression stage, thus often allowing the use of a smaller compressor package than if both permeates were recirculated together to the front of the compressor train inlet.
A further benefit is that a lower total compressor horsepower is required to achieve the same degree of separation and recovery as was previously achievable by recirculating the entirety of the membrane permeate to the front of the compressor train.
Unexpectedly, we have found that, for a given expenditure of compressor horsepower, the process of the invention can result in increased recovery of the second gas compared with the prior art process, without loss of purity of that gas. The second gas can often be recovered at high purity, such as higher than 95% purity, or even above.
Organic components that can be separated from other gases by the process of the invention include saturated hydrocarbons, unsaturated hydrocarbons and substituted hydrocarbons, such as halogenated hydrocarbons. Gases that may be separated from organic components by the process of the invention include nitrogen, methane and hydrogen.
The invention is useful in the treatment of gas streams from petrochemical manufacturing operations, in the separation of off-gas streams from refineries to recover hydrogen and/or LPG, and in the purification of natural gas to remove C3+ hydrocarbons, for example. The invention is particularly useful for the separation of C2+ hydrocarbons from nitrogen.
Such a hydrocarbon/nitrogen stream is typically found in polyolefin manufacturing plants, such as polyethylene and polypropylene plants. As part of the polyolefin manufacturing process, the raw polyolefin product is purged with large quantities of nitrogen to remove unreacted olefins. The resulting olefin/nitrogen off-gas stream can be treated by the process of the invention to recover both the nitrogen and the olefin in purities adequate for return to the manufacturing process.
Any types of compressors may be used to carry out the compression step, and additional compression stages may be included before, after or between the two stages to which the membrane permeates are recycled. The overall pressure change of the gas across the total compression step may be any value consistent with the other requirements of the process, but is preferably below about 1,000 psi.
The membrane separation steps are carried out using membranes selective in favor of the organic component over the second gas. Any membrane that provides such properties may be used. Rubbery polymeric membranes, particularly silicone rubber membranes, are preferred.
Particular parameters that are important in carrying out the process of the invention, and especially with respect to the first membrane separation step, are the organic component concentration in the feed to that step, the feed/permeate pressure ratio across that membrane separation step, and the selectivity of that membrane. As explained in the detailed description of the invention below, the process is more suitable for use, and performs better, if these parameters satisfy certain quantitative inequalities. Specifically, it is preferred that the pressure ratio across the first membrane separation step be more than about 3 and less than about 10. Further, it is preferred that the concentration of the organic component in the feed gas entering the first membrane separation step be above about 10%, or that the selectivity of that step for the organic component over the second gas be no more than about 20, or both.
Recycle of the membrane permeate streams to the compression steps creates a processing loop, from which at least two separated gas streams, either or both of which may be the desired products of the process, are withdrawn. One of these streams is the organic-depleted, second-gas-rich stream, which emerges as a residue stream from the membrane separation steps. The process may often be configured to produce this stream at high second-gas purity, such as 95% purity or above. The other stream is the organic-enriched, second-gas-depleted stream, which may be withdrawn at any convenient point in the processing loop, but typically is taken between the last compression and the first membrane separation steps. This gas may simply be withdrawn as a purge. More typically and preferably, the process includes a cooling or other recovery step between the compression steps and the membrane separation steps from which the organic component is removed at higher purity than its concentration in the loop.
It is an object of the invention to provide an energy- and cost-efficient process for separating gas mixtures containing an organic component and a second gas.
It is an object of the invention to provide an improved process for treating off-gas streams from petrochemical manufacturing processes.
Additional objects and advantages will be apparent from the description of the invention to those skilled in the art.
It is to be understood that the above summary and the following detailed description are intended to explain and illustrate the invention without restricting its scope.